The present invention concerns an improvement for a method of applying gel-coating to seeds of plants.
Seeds of plants (hereinafter simply referred to as seeds) have generally been directly seeded in a field, or previously seeded in a nursery. In the case of sowing them in the nursery, seedlings raised to a certain extent in the nursery are transplanted to the field. In the case of the direct sowing, seeds are liable to suffer from disease injury through sowing. In addition, if the grain size of seeds is so small as in the case of salad seeds for instance, they are often washed away upon rainfall or watering. Furthermore, in the case of the direct sowing, since the seeds are present near the surface soil, they are often damaged being eaten by birds or other animals and, accordingly, development for special techniques has been demanded in recent years as countermeasures therefor.
Recently, machine sowing has often been employed in place of hand-sowing for the reason that hand-sowing requires thinning after the growth of plants and often hinders uniform growing of the plants. However, many restrictive factors are present in the machine sowing and the main factors can include, for example, that seeds are large in size, spherical in shape as much as possible and also uniform in shape regarding an identical kind of seeds and have no peripheral flagellum or arista. Accordingly, all kinds of seeds are suitable to the machine sowing and, for example, those seeds such as of carrot and spinach have to be seeded by hand-sowing in view of their shapes. In addition, it is difficult to sow seeds each by three to four grains even in the machine sowing. Further, since only the stripe sowing is possible in the conventional device, thinning work is unavoidable, which requires much labour as the administrating operation after sowing.
In addition, for the prevention of the diseases injury as one of the countermeasures described above, there have been known a method of immersing seeds in a chemical solution, a method of adhering chemical dressing to the surface of the seeds and a method of applying dry heating treatment. However, the former two methods are not quite satisfactory having the drawbacks that the chemicals do not sufficiently penetrate to the inside of the seeds or the dressing is liable to be defoliated. Further, the latter method involves a problem of the applicability depending on the heat resistant nature of seeds and, if the application conditions are not appropriate, the seeds may be killed.
Further, as the countermeasure for the washaway and damage by the animal eating of the seeds, it has been known to apply a coating material to the surface of seeds thereby increasing the grain size larger into a so-called coated seeds. Since the coated seeds enables easy and accurate machine or hand-sowing, as well as enables the addition of germicide, insecticide, animal repellent, nutrient agent (fertilizer), etc. to the coating material, there have been made various studies and proposals for coated seeds in recent years.
As the basic material for the seed coating, there have been known clays or analogous materials of clay minerals, mixtures of fine sand and paraffin, mixtures of sand and sucrose, etc., as well as the use of gels as the carrier material for the seeds has also been known. The method of using gels as the carrier material for the seeds is disclosed, for example, in Japanese Patent Laid-Open Nos. Sho 55-3796 and Sho 54-49807.
That is, the former literature discloses that an aqueous gel is used as the plant growing culture medium or as the stock culture medium in the case of transporting the plants, for supplying water required during growing or transportation of the plants by the water in the gels with no additional supply of water. It is described that clay type gels, natural gels, synthetic organic gels, etc. are suitably used and they may be molded into an appropriate cubic form such as a small bowl or tray. Further, the latter literature discloses hydroculture of paddy rice plants and describes that an aqueous solution of gelatinelike material which is geled upon cooling (for example, an aqueous warmed solution of polysaccharide) is supplied to seeds thereby coating the seeds and maintaining them in a gel layer, as well as that unexpected displacement of seeds can be prevented during cultivation and water required for the germination and growth of the seeds can be supplied from the water in the gel like that in the former case.
Further, Japanese Utility Model Laid-Open No. Sho 56-20409 discloses that water-insoluble hydrophilic gels, for example, grafting products of methacrylic acid and acrylonitrile are suitable as the gel coating material for seeds, and the seeds can directly be coated with these gel materials and molded into various desired shapes such as tablet, pellet, dice, etc. Further, it is also described that the coating gel also serves for the supply of material other than water such as growth hormone, fertilizer, etc.
Furthermore, Japanese Patent Laid-Open No. Sho 48-85311 describes encapsulated seeds in which seeds and powdery nitrient substances are sealed in a easily water soluble capsule.
However, the known seeds coated with conventional coating material or aqueous gel cannot satisfy three factors, that is, water, temperature and oxygen which are generally considered essential as the conditions for the germination of seeds. That is, although only the two factors, i.e., water and temperature can be satisfied, supply of the oxygen is restricted by the aqueous membranes and, accordingly, reduction in the germination percentage cannot be avoided. Although there has been a method of applying dressing, for example, with calcium peroxide (CaO.sub.2), for supplying oxygen caused by the chemical change thereof, it causes considerable injury to the plants.
If the seeds are stored in a moistened gel lacking in the oxygen supply for a long period of time, the seeds are swollen and cannot be used at all for the purpose of long time storage. Further, encapsulated seeds using readily water soluble capsules as described in Japanese Patent Laid-Open No. Sho 48-85311 cannot be expected to have a long lasting effect since the capsules are easily washed away upon watering after the sowing and the nutrient substances are also washed away rapidly.
The present applicant has already filed an invention concerning gel-coated seeds coated with an aqueous gel having a plurality of fine pores or air bubbles for the sufficient supply of oxygen required for the germination of seeds. However, since the gel coating is water soluble, there has been a problem that the chemicals such as nutrient substances sealed in the coating layer are liable to be runoff.
As have been described above, gel-coating for plant seeds in general can provide useful advantages on one hand but it requires further countermeasures such as runoff useful substances contained in the coating or the coating per se, or for the lack of oxygen caused by the coating on the other hand.
There may be considered further problems to in the case of such gel coating depending on the kind of plant seeds, particularly, regarding the way of sowing them. Such problems will now be described, for example, for the seeds of paddy rice plant which is one of typical crops plants.
It is customary in Japan that seedlings of paddy rice plant after being grown up to some extent in the nursery bed are transplanted to a paddy field, that is, such a transplantation culture is taken place in about 99% or more of the total rice cropping area in Japan, while direct sowing is employed in about 70% of the rice plant cropping area in other districts of the world.
The method of direct sowing employed at present in Japan is generally classified as below:
(a) Direct sowing in well-drained paddy field. PA1 (b) Direct sowing in flooding water. PA1 (c) Direct sowing in water-flooded soils.
While these methods have advantages in common with them, i.e., manpower saving for seedling and rice planting works, irrespective of the difference in the conditions of paddy field and the depth (position) of sowing, they have many problems respectively.
The driect sowing in the well-drained rice field in (a) above is restricted only to mild climate areas, liable to suffer from the effects of climate conditions or suffers from damages, being eaten by animals and also requires a great amount of service water.
The direct sowing in flooding water in (b) above often causes rotting due to the lack in oxygen depending on the sowing position of rice seeds and turned down seedlings, which reduces the percentage establishment. Further, the rice plants are liable to be lodged upon harvesting period.
The direct sowing in water-flooded soil (c) proposed as the improved culture method for (a) and (b) comprises applying a coating to the seeds of rice plant with a mixture of calcium peroxide and sintered plaster and sowing the coated seeds to a thickness of about 10 mm in the water-flooded soil.
Although oxygen can be supplied to the seed due to the chemical change of calcium peroxide for a long period of time, there are the following problems:
(i) Although calcium peroxide releases oxygen in reaction with water, calcium hydroxide is deposited and contaminates the soils.
(ii) Since the seeds are coated in a rotary drum, while a great amount of seeds can be treated, large external force is applied to the seeds to bring about the breakage thereof.
Further, in the conventional direct sowing in water-flooded soil, since pre-treatments such as selection sterilization and immersion of seeds have been conducted prior to the coating and then the coating has been applied with coating material such as calcium peroxide, it has been extremely laborious upon preparing seeds for use in direct sowing in water-flooded soils.
Particularly, in the direct sowing of coating rice seeds in water-flooded soil, there may be a disadvantage that the seeds do not settle in the soils but come to float up in water if their specific gravity is reduced owing to the coated structure.